Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 2Citation - Scopus: 2Magnetic Single-Layer Nanoribbons of Manganese Oxide: Edge- and Width-Dependent Electronic Properties(Royal Society of Chemistry, 2022) Sözen, Yiğit; Topkıran, Uğur; Şahin, HasanIn the present work, the structural, magnetic, and electronic properties of the two- and one-dimensional honeycomb structures of recently synthesized MnO [Zhang et al., Hexagonal metal oxide monolayers derived from the metal-gas interface, Nat. Mater., 2021, 20, 1073-1078] are investigated by using first-principles calculations. Our calculations show that the single-layer 2D MnO crystal has a degenerate antiferromagnetic (AFM) ground state and a relatively less favorable ferromagnetic (FM) state. In addition, the magnetic anisotropy calculations unveil that the easy-axis direction for magnetism originating from unpaired electron states in manganese atoms is normal to the crystal plane. Electronically, while the FM MnO is a direct semiconductor with a narrow bandgap, AFM phases display large indirect bandgap semiconducting behavior. Moreover, the calculations on nanoribbons of MnO reveal that zigzag-edged ribbons display metallic behaviors, whereas armchair-edged nanoribbons are semiconductors. Magnetically, for both zigzag- or armchair-edged nanoribbons, the AFM order perpendicular to the nanoribbon growth direction is found to be favorable over the other AFM and FM orders. Moreover, depending on the edge symmetry and ribbon width, forbidden bandgap values of nanoribbons display distinct family behaviors.Article Citation - WoS: 10Citation - Scopus: 11Single layer PbI2: Hydrogenation-driven reconstructions(Royal Society of Chemistry, 2016) Bacaksız, Cihan; Şahin, HasanBy performing density functional theory-based calculations, we investigate how a hydrogen atom interacts with the surfaces of monolayer PbI2 and how one- and two-side hydrogenation modifies its structural, electronic, and magnetic properties. Firstly, it was shown that the T-phase of single layer PbI2 is energetically more favorable than the H-phase. It is found that hydrogenation of its surfaces is possible through the adsorption of hydrogen on the iodine sites. While H atoms do not form a particular bonding-type at low concentration, by increasing the number of hydrogenated I-sites well-ordered hydrogen patterns are formed on the PbI2 matrix. In addition, we found that for one-side hydrogenation, the structure forms a (2 × 1) Jahn-Teller type distorted structure and the bandgap is dramatically reduced compared to hydrogen-free single layer PbI2. Moreover, in the case of full hydrogenation, the structure also possesses another (2 × 2) reconstruction with a reduction in the bandgap. The easily tunable electronic and structural properties of single layer PbI2 controlled by hydrogenation reveal its potential uses in nanoscale semiconducting device applications.Article Citation - WoS: 63Citation - Scopus: 64Vacancy Formation and Oxidation Characteristics of Single Layer Tis3(American Chemical Society, 2015) İyikanat, Fadıl; Şahin, Hasan; Senger, Ramazan Tugrul; Peeters, François M.The structural, electronic, and magnetic properties of pristine, defective, and oxidized monolayer TiS3 are investigated using first-principles calculations in the framework of density functional theory. We found that a single layer of TiS3 is a direct band gap semiconductor, and the bonding nature of the crystal is fundamentally different from other transition metal chalcogenides. The negatively charged surfaces of single layer TiS3 makes this crystal a promising material for lubrication applications. The formation energies of possible vacancies, i.e. S, Ti, TiS, and double S, are investigated via total energy optimization calculations. We found that the formation of a single S vacancy was the most likely one among the considered vacancy types. While a single S vacancy results in a nonmagnetic, semiconducting character with an enhanced band gap, other vacancy types induce metallic behavior with spin polarization of 0.3-0.8 μB. The reactivity of pristine and defective TiS3 crystals against oxidation was investigated using conjugate gradient calculations where we considered the interaction with atomic O, O2, and O3. While O2 has the lowest binding energy with 0.05-0.07 eV, O3 forms strong bonds stable even at moderate temperatures. The strong interaction (3.9-4.0 eV) between atomic O and TiS3 results in dissociative adsorption of some O-containing molecules. In addition, the presence of S-vacancies enhances the reactivity of the surface with atomic O, whereas it had a negative effect on the reactivity with O2 and O3 molecules.Article Citation - WoS: 60Citation - Scopus: 64Improvement of Optical and Electrical Properties of Ito Thin Films by Electro-Annealing(Elsevier Ltd., 2015) Köseoğlu, Hasan; Türkoğlu, Fulya; Kurt, Metin; Yaman, Mutlu Devran; Akça, Fatime Gülşah; Aygün, Gülnur; Özyüzer, LütfiThe effect of electro-annealing in vacuum and air on the optical and electrical properties of ITO thin films grown by large area DC magnetron sputtering was investigated. Moreover, the performances of the electro-annealed ITO thin films in vacuum and air were compared. Electro-annealing was performed by applying 0.75, 1.00, 1.25 and 1.50 A constant ac current to the ITO thin films. It was observed that the crystallinity of the films was better for the ITO thin films electro-annealed in vacuum. The changes in sheet resistance of electro-annealed ITO thin films with applied currents were detailed. The transmittance of the films increased for both electro-annealing in vacuum and air. A correlation between band-gap and resistivity for all of the electro-annealed thin films was observed.Article Citation - WoS: 12Citation - Scopus: 12High Transmission Through a 90° Bend in a Polarization-Independent Single-Mode Photonic Crystal Waveguide(The Optical Society, 2015) Erol, Adem Enes; Sözüer, Hüseyin SamiWe propose a polarization-independent single-mode waveguide, using a two-dimensional square photonic crystal with a complete band gap. The waveguide is tuned such that both TE and TM modes have the same group velocity and zero group velocity dispersion at the centergap frequency. We also present results for a 90° bend with transmission values of 98% for both modes.Article Citation - WoS: 42Citation - Scopus: 44Growth of Cu2znsns4 Absorber Layer on Flexible Metallic Substrates for Thin Film Solar Cell Applications(Elsevier Ltd., 2015) Yazıcı, Şebnem; Olgar, Mehmet Ali; Akça, Fatime Gülşah; Cantaş, Ayten; Kurt, Metin; Aygün, Gülnur; Tarhan, Enver; Yanmaz, Ekrem; Özyüzer, LütfiIn this work, Cu2ZnSnS4 (CZTS) absorber layers were fabricated using a two-stage process. Sequentially deposited Cu-Zn-Sn thin film layers on metallic foils were annealed in an Ar + S2(g) atmosphere. We aimed to investigate the role of flexible titanium and molybdenum foil substrates in the growth mechanism of CZTS thin films. The Raman spectra and X-ray photoelectron spectroscopy analyses of the sulfurized thin films revealed that, except for the presence of Sn-based secondary phases, nearly pure CZTS thin films were obtained. Additionally, the intense and sharp X-ray diffraction peak from the (112) plane provided evidence of good crystallinity. Electron dispersive spectroscopy analysis indicated sufficient sulfur content but poor Zn atomic weight percentage in the films. Absorption and band-gap energy analyses were carried out to confirm the suitability of CZTS thin films as the absorber layer in solar cell applications. Hall effect measurements showed the p-type semiconductor behavior of the CZTS samples. Moreover, the back contact behavior of these metallic flexible substrates was investigated and compared. We detected formation of cracks in the CZTS layer on the molybdenum foils, which indicates the incompatibility of molybdenum's thermal expansion coefficient with the CZTS structure. We demonstrated the application of the magnetron sputtering technique for the fabrication of CZTS thin films on titanium foils having lightweight, flexible properties and suitable for roll-to-roll manufacturing for high throughput fabrication. Titanium foils are also cost competitive compared to molybdenum foils. © 2015 Elsevier B.V.Article Citation - WoS: 19Citation - Scopus: 25Large Energy Gaps in Ca C6 From Tunneling Spectroscopy: Possible Evidence of Strong-Coupling Superconductivity(American Physical Society, 2007) Kurter, Cihan; Özyüzer, Lütfi; Mazur, Daniel; Zasadzinski, John F.; Rosenmann, Daniel; Claus, Helmut; Hinks, David G.; Gray, Kenneth E.Tunneling in Ca C6 crystals reproducibly reveals superconducting gaps Δ of 2.3±0.2 meV that are ∼40% larger than reported earlier. In an isotropic s -wave scenario, that puts Ca C6 into the class of very strongly coupled superconductors, since 2Δ k Tc ∼4.6, implying that soft Ca phonons are primarily involved in the superconductivity. This conclusion explains the relatively large Ca isotope effect found recently for Ca C6, but it could also signal a strong anisotropy in the electron-phonon interaction.Article Citation - WoS: 31Citation - Scopus: 30Absence of Pseudogap in Heavily Overdoped Bi2sr2cacu2o8+? From Tunneling Spectroscopy of Break Junctions(Institute of Physics Publishing, 2002) Özyüzer, Lütfi; Zasadzinski, John F.; Gray, Kenneth E.; Kendziora, Christopher A.; Miyakawa, NobuakiWe report tunneling spectroscopy of superconductor-insulator-superconductor break junctions on heavily overdoped Bi2Sr2CaCu2O8+δ with Tc = 56 K. At T ≪ Tc, the junction conductances display well-defined quasiparticle peaks at ±2Δ and in some cases a Josephson current at zero bias. Gap values as small as Δ = 10.5 meV have been observed leading to 2Δ/kTc near the BCS limit for dx2-y2 pairing. Temperature dependence of the gap magnitude, Δ(T), follows the BCS relation and both the quasiparticle gap and Josephson current vanish for T > Tc. Above Tc, the tunneling conductance shows a flat background without any indication of a pseudogap near the Fermi level.Article Citation - WoS: 14Citation - Scopus: 14High Energy Secondary Peak Structure in Tunneling Spectra (hump) as Possible Magnetic Pseudogap(Elsevier Ltd., 2000) Zasadzinski, John F.; Özyüzer, Lütfi; Miyakawa, Nobuaki; Hinks, David G.; Gray, Kenneth E.It is demonstrated that tunneling spectra in various high Tc cuprates display generic features. The principal conductance peaks in superconductor-insulator-normal metal (SIN) junctions indicate the superconducting gap in the density of states (DOS), Higher energy features include a dip and hump structure with a strength that is asymmetric in bias voltage. The dip and hump features follow the doping trends of the superconducting gap, Δ, with a rough scaling as ∼2Δ and ∼3Δ respectively. Tunneling spectra in underdoped Bi2Sr2CaCu2O8+δ display a more pronounced hump feature suggestive of a second gap in the DOS. It is observed that the hump feature in the tunneling density of states is consistent with other experimental observations of the so-called high energy pseudogap which may have magnetic originsArticle Citation - WoS: 3Citation - Scopus: 3Tunneling Spectroscopy of Heavily Underdoped Crystals of Bi2sr2cacu2o8-?(Elsevier Ltd., 2000) Özyüzer, Lütfi; Zasadzinski, John F.; Miyakawa, Nobuaki; Kendziora, Christopher A.; Sha, J.; Hinks, David G.; Gray, Kenneth E.Crystals of Bi2Sr2CaCu2O8+δ with optimal Tc=95 K have been underdoped using two different methods and the superconducting gaps have been obtained by tunneling. In some cases, three different tunneling geometries have been utilized: point contact, STM and break junctions. The first doping method involves control of the oxygen content by annealing in various partial pressures of oxygen. These crystals exhibit a narrow spread of gap values over a wide doping range from overdoped (Tc=56 K) to underdoped with Tc=70 K. However, for underdoped crystals with Tc midpoints in the range 25 K - 63 K, there is a dramatic increase in the spread of gap values which may signal the development of static phase separation of either chemical or electronic origin. To avoid possible chemical phase separation, we have explored another doping procedure which incorporates Dy substitution on the Ca site. These crystals exhibit a relatively narrow superconducting transition width and some preliminary tunneling spectra will be presented.